Update flake8 requirement from <5,>=4 to >=4,<7

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@4.0.0...6.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -69,7 +69,7 @@ def read(path):
               'zc.customdoctests>=1.0.1,<2',
               'createcoverage>=1,<2',
               'stopit>=1.1.2,<2',
-              'flake8>=4,<5',
+              'flake8>=4,<7',
               'pytz',
               # `test_http.py` needs `setuptools.ssl_support`
               'setuptools<57',